1. Field of the Invention
The present invention relates to a recording material and, more particularly, to a recording material such as a pressure-sensitive copying paper, a heat-sensitive recording paper, a light-sensitive recording sheet, etc., capable of providing color images which have excellent light resistance.
2. Description of the Prior Art
A large number of systems have been suggested for recording information using as a stimulus energy such as pressure, heat, light, electricity, etc. However, quite few systems have in actuality been put into practice.
The recording system employed for commercially available pressure-sensitive copying papers, heat-sensitive copying papers and electrothermic heat-sensitive recording papers utilizes the phenomenon that colors are formed by bringing a dye precursor or an electron-donating material (also called a "color former", which is a substantially colorless organic compound capable of reacting with an electron-accepting material, to be described hereinafter, to form a color) into contact with an electron-accepting material (also called a "color developer", which is a material acting as a Bronsted acid or a Lewis acid, such as clay minerals (e.g., activated clay, etc.); organic acids (e.g., phenols, phenolic resins, organic carboxylic acids, organic sulfonic acids, etc.); metal salts of phenols, organic carboxylic acids or organic sulfonic acids (e.g., salts of Al, Zn, Ni, Sn, etc.); inorganic acids (e.g., hydrohalic acids, boric acid, silicic acid, phosphoric acid, sulfuric acid, nitric acid, perchloric acid, etc.); halides of Al, Zn, Ni, Sn, Ti, B, etc.) due to the application of pressure or heat or of electrical energy (with at least one of the color former and the color developer forming the contact as a solution, a liquid or a gas).
Of light-sensitive recording sheets, only a diazo light-sensitive paper is generally used in addition to silver salt light-sensitive materials as an inexpensive light-sensitive recording sheet. With respect to light-sensitive recording sheets, too, systems combining a dye precursor of an electron-donating material with a compound capable of forming an electron accepting material which is a solid acid when exposed by light have been suggested and are described in Japanese Patent Publications 24188/63 corresponding to U.S. Pat. No. 3,140,947, 10550/70 corresponding to U.S. application Ser. No. 533,869, filed Mar. 14, 1966, 13258/70 corresponding to U.S. application Ser. No. 502,498, filed Oct. 22, 1965, 6212/74, 28449/74, Japanese Patent applications (OPI), 80120/75, 87317/75, 126228/75, etc. However, these systems have not been put into practice.
Dye precursors capable of forming a color when brought into intimate contact with an electron accepting material have heretofore been studied mainly as color formers for pressure-sensitive copying papers. The properties required for color formers to be used for pressure-sensitive copying paper vary over a wide range. The main requirements as to the properties of color formers are that they (1) be substantially colorless; (2) be capable of forming color almost instantly when in contact with an electron accepting material; (3) be soluble in a certain organic solvent; (4) do not sublime; (5) do not undergo decomposition or coloration in the air due to light, heat or humidity before being brought into contact with an electron accepting material; (6) form a dye with the help of an electron accepting material, which dye is resistant to light, heat or humidity; (7) be non-toxic and do not cause environmental pollution; (8) be inexpensive to synthesize on an industrial scale; and the like.
At present, there are no color formers that satisfy all of these requirements, and the fact is that two or more color formers are used in combination or a specific electron accepting material is selected to compensate to some extent for the properties in which the color formers are deficient.
Color formers for pressure-sensitive copying papers are required to possess so many properties, each of which should be exhibited to a high degree, that color formers which can be used for pressure-sensitive copying papers are naturally expected to be also usable as dye precursors for the abovedescribed heat-sensitive recording papers, electrothermic heat-sensitive recording papers, light-sensitive sheets, etc., which utilize coloration by the reaction between an electron donating material and an electron accepting material. In fact, dye precursors used in commercially available heat-sensitive recording papers and electrothermic heat-sensitive recording papers are those used as color formers for pressure-sensitive copying papers.
In addition, dye precursors for light-sensitive recording sheets described in the foregoing patents are mostly known as color formers for pressure-sensitive copying paper. Furthermore, also the intentions are to apply color formers for pressure-sensitive copying paper to ultrasonic wave recording papers (French Pat. No. 2,120,922), electron beam recording materials (Belgian Pat. No. 7,959,986), electrostatic recording papers (Japanese Patent Publication 3932/74), formation of dye images on light-sensitive printing plate materials (Japanese Patent Application (OPI) 12104/73), color former inks (Japanese Patent Publication 10766/72), typewriter ribbons (Japanese Patent Application (OPI) 3713/74), inks for ball point pens (Japanese Patent Application (OPI) 83924/73), crayons (U.S. Pat. No. 3,769,045), etc.
A large number of patents have issued on color formers for pressure-sensitive copying papers. However, there are known no color formers that satisfy the above-described requirements (2), (5) and (6), much less satisfy all eight requirements set forth above. Crystal violet lactone having the structural formula (A): ##STR2## at present used as a blue color former satisfies requirements (2) and (5), but does not satisfy requirement (6). Therefore, this compound is used in combination with N-benzoylleucomethylene blue having the structural formula (B): ##STR3## which satisfies requirement (6) but which does not satisfy requirements (2) and (5).
However, since the coloration hue of these two color formers of the formulae (A) and (B) above differs from each other, it is necessary to add another color former to adjust the coloration hue. Therefore, this process is economically disadvantageous and, in addition, there is the defect that the color formers form a color when exposed to light befor use.
As red and black color formers, fluoran color formers as represented by the following formula (C): ##STR4## can be used.
These color formers have the disadvantage that the color density, hue and light resistance vary greatly depending upon the kind of electron accepting materials, especially between activated clay and a phenol resin with which they are used. Ideal electron accepting materials have not been discovered. For example, activated clay provides poor light resistance, although it provides a high color density, whereas a phenol resin generally provides good light resistance but provides a low color density.
Some color formers as used for pressure-sensitive copying papers are also used as color formers for heat-sensitive recording papers and electrotheric heat-sensitive recording papers. In these cases, however, problems with respect to light resistance and fog prevention before coloration (stain in the background) exist. The main reason therefor is that requirements (5) and (6) required for color formers for pressure-sensitive copying papers are not fully satisfied.
A light-sensitive recording sheet can be obtained by coating a compound capable of forming an electron accepting material (e.g., a hydrogen halide like hydrogen chloride, hydrogen bromide, hydrogen iodide, etc.; a carboxylic acid; a sulfonic acid; a phenol), such as organic halides (e.g., carbon tetrabromide, .alpha.,.alpha.,.alpha.-tribromoacetophenone, hexachloroethane, iodoform, 2-tribromomethylpyridine, trichloromethylsulfonylbenzene, etc.; o-quinonediazide compounds as described in Japanese Patent Application (OPI) 12104/73; phenol esters of a carboxylic acid or a sulfonic acid capable of undergoing a photochemical Fries rearrangement), and an electron donating dye precursor (color former for a pressure-sensitive copying paper) on a support of a paper or a synthetic resin film together with a suitable binder. However, conventional dye precursors have the defects that the resulting color images have poor light resistance, and that the occurrence of fog before coloration is serious while the color density in the light-irradiated areas is low.
In addition, application to ultrasonic wave recording papers, electron beam recording papers, electrostatic recording papers, light-sensitive printing plate materials, color former inks, typewriter ribbons, inks for ball point pens, crayons, etc., involves the problems that the density and light fastness of the resulting color images are insufficient, and that fog is formed during storage before use and coloration upon use is poor, since conventional dye precursors (color formers) do not sufficiently satisfy, in particular, requirements (2), (5) and (6) of the requirements for pressure-sensitive copying paper as described before.